It all started in high school art class when Cozzens was a junior. (Footnote: Cozzens went to the same high school, York Suburban, as Pinterest co-founder Evan Sharp.) She used the social media site to bookmark, or "pin," images of tattoos she wanted to draw in class. She added other pinboards along the way, organized under "Style" or "Hair" or "Logos."

Then, last November, a Pinterest marketing firm in California tapped her to point people to its clients. Now she's bringing in income on the side.

As for how she became a Pinterest star with a massive following, Cozzens isn't sure.

"That's the one question everyone asks me, and I have no idea," she said. "I guess they like my style and the things that I have on there. Um. I really couldn't tell you though, because I don't know."

Brendan Lowry has a hunch.

He's the marketing director of the Philadelphia-based Curalate, which has made its name making Pinterest work for big brands.

"So, there are a few users who kind of got lucky," Lowry said. "Because they were curating interesting content, whenever a new user signed up, they would automatically follow specific people [featured by Pinterest], and I assuming that's kind of how Brittany got her start."

Three million followers is still impressive, Lowry says. There are only about 20-30 top-shelf influencers in what Lowry calls a new marketing ecosystem — where people want to speak online with pictures, not words.

"And among this is a new class of these influencers or tastemakers who are really, really good at modern-day storytelling," Lowry said. "People like Brittany have shown this ability to tell stories visually — and that's where these big brands and these vendors are coming in and recognizing this and rewarding them for it."

Lowry says many companies are starting to do this on their own. You can track a very specific return-on-investment for Pinterest influencers, he says.

Cozzens, whose account outnumbers many of the country's biggest brands, doesn't go out of her way to talk about it much.

After graduation, her goal is to move to the South and one day be an art director at an ad agency.